Hydraulic systems rely upon flow of a hydraulic fluid, typically an oil or water based liquid, to achieve a desired effect. One example of such a system is a hydropneumatic suspension of a vehicle, in which road wheels are mounted on respective hydraulic actuators, and springing is provided by a hydropneumatic suspension module containing a springing medium, such as nitrogen gas. Suspension modules are typically in the form of a sphere having a diaphragm separating hydraulic fluid from a suspension gas under pressure. Working of the suspension causes movement of hydraulic fluid to and from the sphere. Such suspension systems are well known and need not be further described here.
As the hydraulic fluid moves to and from the suspension module, some means of damping must be provided to reduce the amplitude of suspension movement. Orifices and spring loaded relief valves have been proposed to allow suspension characteristics to be tuned for optimized ride and handling of the vehicle. The characteristics for bump and rebound are usually different, so that typically the fluid flow path in the bump (upward) direction of a wheel is different from that of the rebound (downward) direction of a wheel.
Some kinds of vehicle, particularly agricultural and construction vehicles, are designed to operate both on and off highway. The respective ranges of suspension movement are significantly different, so that a suspension optimized for on-highway use may be too hard for off-highway use. Conversely a suspension capable of accommodating wheel deflections in off-highway use may be uncomfortably soft in on-highway conditions. In either case the speed of travel of a vehicle may be severely limited if used on terrain for which the suspension is not optimized. A driver selectable alternative increases expense and complication, and is preferably avoided.